Can I skip a step and come back to it?

Every step is skippable, and so is the whole wizard. Skipping leaves a data-health flag rather than a wrong number: the statement will tell you that marketing spend is incomplete, or that products have no cost, instead of quietly treating the missing figures as zero.

That is the design rule behind the whole product. A missing cost is reported as missing, never assumed to be nothing, because a margin computed on absent data is worse than no margin at all.